1. Field of the Invention
The present invention relates to a data transmission system in a two-wire subscriber's line using a metallic cable, and more particularly to a start control system for transmission control of LSIs provided in a subscriber side terminal apparatus and in an exchange side apparatus to which apparatuses a time sharing compression transmission system called "a ping-pong transmission system" is applied.
2. Description of the Prior Art
Conventionally, a time sharing compression transmission system, called a "ping-pong transmission system", was employed for two-way digital transmission between two apparatuses using a two-wire subscriber's line composed of a metallic cable such as the existing telephone line.
This system is adapted to share a transmission path in a time sharing manner, and transmit a digital data burst signal only in a single direction, upwardly or downwardly, and thereafter to alternately repeat such a transmission for two-way transmission of the digital data signal.
The following references, for example, describe the principles of prior art ping-pong transmission systems: (1) H. R. Brown and G. C. Mason; "Services for the Emerging ISDN", ICC '81, Colo. (2) J. H. M. Hardy and C. E. Hoppitt; "Access to the British Telecom ISDN", ICCC '82 (3) Jan Meyer, Terje Roste, and Roald Torbergsen "A Digital Subscriber Set" IEEE TC, Vol COM-27, No. 7 July 1979.
An exemplary arrangement, to which such a prior ping-pong transmission system is applied, will here be described with reference to FIG. 1. FIG. 1 is a block diagram illustrating digital data communicatin between a subscriber side digital telephone set 8 and an exchange side digital subscriber circuit 10 connected via a metallic two-wire telephone line 9.
As shown in FIG. 1, the subscriber side includes a handset (HS) 1 for ordinary telephone call;
a speech network (SN) 2 for two-way transmission of an analog speech signal;
a CODEC (coder and decoder) 3 for bidirectionally converting between an analog speech signal and a digital signal;
a slave LSI (i.e.-line subscriber interface) (S) 4 for controlling the pin-pong tansmission, a data terminal (DTE) 5 for transmitting/receiving digital data;
a data terminal interface (DTI) 6 to match the data terminal 5 and the slave LSI 4 to each other, and
a central processing unit (CPU) 7 for controlling the operations of respective components of the digital telephone set 8.
While, the exchange side includes, although neglected in part, a time sharing exchange switch (TSW) 17, a central control unit (CC) (i.e.-subscriber line interface circuit) 15, and a digital subscriber circuit (SLIC) 10. The digital subscriber circuit 10 has n master LSIs (M) 11a... 11d for controlling the ping-pong transmission, which are controlled by a CPU 13 via a control bus 12. Moreover, in the same figure, designated at 14 is a control bus for permitting the station side central control unit (CC) 15 to control the digital subscriber circuit 10, and 16 is a highway (HW) through which an exchange signal to be transmitted passes.
In succession, transmission codes and a ping-pong period of the ping-pong transmission system, and operation and a start system of a prior art ping-pong transmission LSI will be described.
There are a wide variety of transmission codes for use in the ping-pong transmission system. Here an AMI code is assumed and further assumed is a ping-pong transmission system of 144 kb/s comprising speech data (8 bits) B1, data (8 bits for data terminal, etc.) B2, and control (2 bits) D. In addition, the AMI code typically includes a frame inserted thereinto in need of mutual synchronization independently of transmission data. FIGS. 2(A)-(B) exemplarily illustrate the arrangement of the AMI code, wherein tf is a frame interval; td is a data interval (8 bits+8 bits+2 bits)(B+B+D), and t is a transmission bit period of the AMI code. A DC balancing bit is inserted into the rear portion of the AMI code depending on the data arrangement. FIG. 2 (A) illustrates the arrangement of this AMI code, which will hereafter be indicated by a symbol of FIG. 2 (B) as one burst.
FIG. 3 illustrates a signal arrangement during the ping-pong transmission, wherein S1 and S3 indicate data from the slave LSI to the master LSI, and S2 and S4 indicates data from the master LSI to the slave LSI. A time interval Tp between the data S2 and S4 is the ping-pong period.
The ping-pong transmission is divided broadly into two main apparatuses: one taking the initiative in transmission timing and the other following after the former. The apparatus taking the initiative here is the master LSI on the exchange side while the follow-up apparatus is the slave LSI on the side of the digital telephone set. For the exchange, digital exchanges take the lead at present, which are adapted, from the viewpoint of processing speech data, to undergo time-sharing switching in, synchronism with the Nyquist frequency of 8KHz. On the other hand, the digital telephone set is to only receive data from the exchange and transmit transmission data on this side to the partner. Accordingly a ping-pong transmission in synchronism with 8 KHz on the exchange side is effected.
Hereupon, the above description assumed the LSIs effecting the ping-pong transmission to be the master LSI on the exchange side while the slave LSI on the side of the digital telephone set, and they will hereinafter be referred to as a MLSI and a SLSI, respectively. But, the actual MSLI and SLSI may have the same architecture and hence be switchable from the outside to be a master or a slave.
In succession, operations of these MLSI and SLSI will be described. The MLSI takes, as described previously, the initiative in transmission timing so that the MLSI first transmits data to the SLSI and then waits for any data from the SLSI. This state of the MLSI of waiting for data from the SLSI here means a state thereof of not transmitting the AMI burst and also being capable of receiving data from the other side. The MLSI further, after effecting the above ping-pong operation for a prescribed time interval, again performs the operation just described above and repeats these operations thereafter. The SLSI maintains a state of waiting for receiving data from the MLSI, and transmits data to the MLSI immediately after completing the data reception. Thereafter, the SLSI again maintains the waiting state for data reception.
The sequences of both prior art LSIs for controlling the start of data transmission after establishing communication will be described with reference to FIG. 4. As shown in the same figure, the vertical direction represents a flow of time passing downwardly with time points t1 to t5 of transmitting and receiving respective signals. Moreover, a power-down mode means one not transmitting data while a power-up mode means one transmitting data. The power down mode is here a mode provided for reducing power consumption upon non-data transmission in a state of the system connected to the transmission line.
Describing this in further detail, the digital telephone set, etc., including the SLSI contained therein are often powered from the exchange side, and hence power consumption to be applied to of the SLSI is limited. The line impedance of a telephone circuit is typically 110 .OMEGA., and many methods to transmit the AMI code are arranged as illustrated in FIG. 5. In the same figure, designated at 61 is the MLSI or SLSI, Q1 and Q2 are respectively NPN transistors, PT is a pulse transformer, S1 and S2 are respectively output terminals of the LSI to drive the transistors Q1 and Q2, and VDD is a power source. Power consumption in the LSI is typically much lower than the power to drive the pulse transformer, and hence it is rather advantageous for reducing the power consumption to have no output from Sl and S2 when the ping-pong transmission is not performed. This mode is the power-down mode illustrated in FIG. 4, which controls the outputs from Sl and S2 to stop them from the outside of the LSI. Against this, the power-up mode is one enabling Sl and S2 to output any signal, under which mode the MLSI and SLSI are usually employed.
Here, the MLSI and SLSI are under the power-down mode before starting the control of data transmission. When a start switch (not shown) in the MLSI is turned on, the MLSI is switched to the power-up mode, and transmits a start instruction signal A to the SLSI (t1). This signal A is not specific but is a burst signal according to the AMI code shown in FIG. 2. The SLSI is judged to receive the start signal by permitting a 8 KHz frame signal (involved in tf of FIG. 2) to be extracted from the signal A. The SLSI further, upon receiving this signal A, counts the frame signal and informs the outside of the reception of the start instruction signal when the count reaches a prescribed value. 2 bits for control in the AMI code are employed as a means for such an informing operation, the informing operation being executed by transmitting a command to an external CPU, etc. The CPU etc., so receiving the command, release the SLSI from the power-down mode externally and switches it to the power-up mode. The SLSI in the power-up mode transmits an answer signal B to the MLSI in the reception waiting state, in synchronism with the transmission timing (8KHz) of the MLSI (t3). The answer signal B is a burst signal according to the AMI code shown in FIG. 2 similar to that of the start instruction signal A. A time interval T1 in the same figure is a time difference (t3-t2) of processing from the time the SLSI informs the outside of reception of the start instruction to the time the external CPU etc. release to the SLSI from the power-down mode. The MLSI, upon receiving the answer signal (t4), counts the frame signal in the same manner as the SLSI, and judges, after (t5) a prescribed time interval (t2), that the start of the control is established, and transmits data C, and thereafter performs the known ping-pong operation.
Conventionally, it was possible as described above, to start the control by issuing a start instruction signal from the master to the slave. This is due to the above described prior art system simply effecting one-way (master.fwdarw.slave) ping-pong start control. There is however, a need for effecting such a start control oppositely, i.e. from the slave to the master.
In fact, these master and slave LSI's are simply different in that one transmits the AMI code to the partner in synchronism with an external synchronizing signal (8 KHz in the present case) or the other transmits the AMI code to the partner in synchronism with the AMI code transmitted. The former is the master and the latter the slave, which latter slave can functionally transmit such a start instruction signal as in the master. However, the master, if receiving the start instruction signal, is restricted in its transmission timing by a synchronizing signal provided externally, and hence answer signal timing from the master competes with the start signal from the slave on a transmission line to result in difficulty of assuring a start sequence from the slave to the master.